During conventional fabrication of microcircuit components, patterns are successively transferred from masks to resist layers on a wafer. In order to build a useful microcircuit component, each successive pattern must be accurately aligned with previously transferred underlying patterns. As the linewidth of microcircuits gets smaller, alignment accuracy must be correspondingly improved. Furthermore, the number of alignment steps needed to fabricate a single microcircuit component has been increasing, thus making an automatic alignment system highly desirable.
One method of transferring a pattern from a mask to an optically sensitive wafer surface is by optical projection printing. This technique has the advantage that the pattern on the mask can be scaled up in size for ease in fabricating the mask. A projection system then demagnifies the enlarged pattern to the desired size during a subsequent projection printing step. An accurate alignment system for projection printing which can be readily automated is described in my U.S. Pat. No. 4,232,969.
A disadvantage with projection printing is that optical focussing elements are needed. This limits the wavelength of the light which can be used for projection printing since suitable focussing elements do not exist at short wavelengths, such as for example in the soft x-ray region of about 6 .ANG. to 25 .ANG. in wavelength. This region is of particular interest because lithographic resist materials are being developed which are sensitive to x-rays in this region. Such shorter wavelengths are generally of interest for lithography because they promise the possibility of improved resolution, smaller linewidths and more dense, compact, faster and possibly cheaper microcircuits.
Since projection printing currently cannot be used for x-ray lithography, proximity printing is contemplated instead. This method places the mask very close to but not in contact with the wafer because contact would cause damage to the wafer surface and the mask. The proximity mask is then flooded with actinic radiation. While proximity printing is a generally well known lithographic technique, highly accurate alignment techniques still do not exist for this method and none of the known alignment techniques for proximity printing systems are readily automatable.
Prior art alignment systems for proximity printing generally involve the use of a split field alignment microscope. A person visually observes through a microscope an alignment mark on the wafer and an alignment mark on the proximity mask and then adjusts the relative position of the proximity mask and wafer until the marks are aligned. One problem is that the marks are not in the same object plane (typically they are separated by 20-60 .mu.m) and microscopes typically can be focussed only on one plane at a time. One solution is to use a low numerical aperture optical system so that there is a large depth of field, but this results in lower resolution and consequently lower alignment accuracy. Another solution is to use a bifocal optical system. Such a system is described, for example, by A. White in "Simple bifocus element for microscope objectives", 16 Appl. Optics 549 (1977). Unfortunately, bifocal elements have reduced contrast and any microscope which uses visible light has resolution limits imposed by the wavelength of the light employed. Automation of such systems has not been particularly successful because of the inherent complexity involved in aligning (and consequently matching) two images electronically.